D. J. Hearn

Mapping using the Tsyganenko long magnetospheric model and its relationship to Viking auroral images

The Tsyganenko long magnetospheric model (1987) has been used in conjunction with ultra-violet images taken by the Viking spacecraft to investigate the relationship of the auroral distribution to different magnetospheric regions. The model describes the large-scale structure of the magnetosphere reasonably well for dipole tilt angles near zero, but it appears to break down at higher tilt angles. Even so, a wide variety of auroral configurations can be accurately described by the model. It appears that the open-closed field line boundary is a poor indicator of auroral arc systems with the possible exception of high-latitude polar arcs. The auroral distribution typically called the oval maps to a region in the equatorial plane quite close to the Earth and can be approximately located by mapping the model current density maximum from the equatorial plane into the ionosphere. Although the model may break down along the flanks of the magnetotail, the large-scale auroral distribution generally reflects variations in the near-Earth region and can be modeled quite effectively.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.

Comparison of UV optical signatures with the substorm current wedge as predicted by an inversion algorithm

Optical images of the auroral bulge as seen by the Viking UV imager were compared in several cases with the substorm current wedge (SCW) upward and downward field-aligned currents (FAC) whose positions were determined using the inversion algorithm based on the substorm-related magnetic variations observed at midlatitudes. With reasonable accuracy (better than 0.5 hours MLT) the estimated longitudes of the upward FAC generally pointed to the surge or to the brightest luminousity region in the western half of the bulge. The latter feature may imply a more complicated structure of the net FACs than can be described by the simple substorm current wedge scheme. Similarly, the estimated positions of the downward FAC pointed close to the eastern termination of the bulge. The associated optical signatures of this current system ranged from the well-defined emission depletion regions to new auroral intensifications. The downward current appears to correspond in some cases at least with the division between the morning sector portion of the double oval and the nightside portion connected more directly to the substorm bulge. The results in general confirm the expected association between the auroral bulge and the SCW, as well as showing a reasonably good results from the inversion algorithm based on midlatitude magnetic observations. Our results, however, also indicate that one must be careful in interpreting the apparent motion of SCW-related field-aligned currents inferred from midlatitude observations in terms of a true westward or eastward expansion of the SCW or of the auroral bulge. The observed changes may instead sometimes be related to the redistribution of the net FACs within than a shift or expansion of the simple current system.

Interpretation of optical substorm onset observations

Abstract The ionospheric location of substorm onset is generally found to be at the most equatorward arc in the poleward portion of the diffuse aurora. The observation that most activity occurs in this region provides a reference from which the source region in the magnetotail may be assessed. This reference can be examined in two ways. First, magnetic field mappings of these onset locations to the equatorial plane suggest that the onset is associated with processes quite near the Earth. For example, for 14 cases the average GSM X value was found to be ≈ −7.8 R E . However, this identification is based on a static magnetic field model and while these results are consistent with some earlier findings there is not sufficient confidence in this technique to discriminate between topological regions in the magnetotail. A second way to examine the ionospheric onset location is in relation to the open/closed field line boundary. It is evident from Viking satellite images that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and active periods. There is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore unrelated to the open/closed field line boundary, a result consistent with some (but not all) near-Earth mechanisms but only under some conditions with the distant tail boundary layer theory.

The auroral distribution and its mapping according to substorm phase

Abstract An attempt is made to reconcile two competing views as to where the auroral distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a near-Earth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic 1–10 keV electrons and 0.1–1 keV structured electrons. The ionospheric local time sector between 13 and 18 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an auroral distribution specific to the substorm recovery phase. These auroral observations provide a means of inferring important information concerning magnetospheric topology.

Freja UV imager observations of spatially periodic auroral distortions

Freja UV imager measurements of distortions of discrete arcs within the auroral distribution reveal a number of examples of periodic intensifications. Analysis of these features reveals a clear peak at 152 km separation between the intensifications with a less well defined peak at approximately twice that value. No observations at separations less than 100 km were found. Theories relating such features to shear flow regions suggest a relationship between wavelength and shear layer width. A statistical analysis indicates that the product of the wave number and the arc system width yielded a value of .7 which is consistent with theoretical predictions. The periodic features were found to occur throughout the afternoon/midnight sector with no apparent dependence on MLT. As well they appeared at a variety of locations within the auroral distribution implying the common existence of shear layers.

Satellite observations of polar arcs

Abstract The increased probability of observing polar arcs during periods of northward IMF has tended to obscure their significance in terms of magnetospheric topology because of the presumed ‘inactive’ state of the magnetosphere. However, satellite imaging has shown that these high latitude features are quite dynamic both in their intensity and spatial variations. The overall morphology of the high latitude aurora has been described by a variety of imaginative terms, but its primary optical characteristic is of a polar arc(s) extending between the dayside and nightside auroral distribution on one or both of the dawn/dusk sides of the high latitude region. This large scale morphology is controlled by the azimuth angle of the IMF and the predominant configuration is one wherein the region between the polar arc and the normal auroral distribution is filled with low intensity diffuse emission. Simultaneous particle and electric field measurements show this region exhibits a closed field line character with predominantly sunward flowing plasma. These large scale polar arcs are connected (in either a diffuse or discrete fashion) to the nightside auroral distribution with essentially equal probabilities, but exhibit a clear peak near 12 MLT. This dayside connection is commonly associated with isolated high latitude features poleward of the normal auroral distribution which probably represent processes occurring on the front surface of the magnetotail poleward of the cusp. The existence of polar arcs is not always controlled by substorm activity: polar arcs can maintain their form and position well past expansion phase suggesting that they represent a fundamental boundary in the magnetosphere which is not modified by even large substorms.

Dayside Aurora Poleward of the Main Auroral Distribution: Implications for Convection and Mapping

The morphology of high latitude polar arcs is used in conjunction with mapping considerations to infer source regions for convection cells during quiet time conditions. A simple set of rules are presented to aid a researcher in mapping from the high latitude ionosphere into a relatively closed magnetosphere. It is suggested that the multi-cell convection (> two cells) is not a result of large scale magnetospheric convective changes but rather is due to relatively localized low latitude boundary layer instabilities such as the Kelvin Helmholtz in the deep tail. For IMF By 0 in the dusk sector) may be on closed field lines mapping to the flanks of the magnetotail. Changing the flow in that region could give rise to a third clockwise cell in the dawn sector or a well defined anticlockwise cell in the dayside dawn ionosphere with a “turbulent” flow pattern on the nightside. The frequent observation of two arcs embedded within a main sun-aligned structured can also be interpreted by this method. Examples are given as to how each situation could arise.

Oval intensification event observed by STARE and Viking

Optical emissions and electron flow patterns in the auroral oval evening sector have been investigated in the 30-min interval prior to a major auroral intensification. The STARE radar and Viking UV image data were combined to give a dynamical picture of the auroral ionosphere just to the west of the intensification region which was bordered on its eastward side by the foot of a high-latitude polar arc. Half an hour prior to the main intensification an auroral arc appeared equatorward of a band of eastward electrojet, which in turn was equatorward of another discrete arc system. The appearance of the new arc system was accompanied by a second eastward electrojet further equatorward. Within minutes of this appearance the older electrojet and arc system faded and the new system moved poleward to replace it. This precursor activity was followed by a major intensification, a substorm, to the east of the foot of the high-latitude polar arc. It is speculated that the precursor is linked to changes in the current distribution in the near-Earth nightside magnetosphere prior to the substorm onset.